Parasites Join Forces, For Better or Worse

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Tiny parasites such as bacteria and viruses may battle for
control of precious bodily resources, but they can also open the
door for additional invasions by their "friends." A new study
finds that interactions among many parasites in the same host
body can dramatically boost chances for new infections or, in
other cases, lessen the odds.

Not only did the parasites
in the study all interact with each of the other parasites, but
they also varied wildly in terms of impact within the mouse-like
voles that acted as hosts – generating five times greater risk of
infection in one case involving two parasites, and a reduction of
infection risk to just 15 percent in another case. [ The
10 Most Diabolical and Disgusting Parasites ]

This marks the first long-term study to track parasite
interactions within the same individuals in the wild, so that
researchers could rule out the effect of other factors, such as
seasonal changes. Voles proved ideal in this case, because of
their short life spans that range from just 3 to 6 months.

"Because we followed a large number of individual animals over
the course of their lives, we were able to pinpoint the
time an animal acquires an infection and examine what factors
influence susceptibility," said Sandra Telfer, a parasite
ecologist at the University of Liverpool in England.

Researchers drew blood samples from almost 6,000 voles about
every four weeks, using four different populations over several
years. They focused on cowpox
virus and a protozoan, or single-celled, parasite called
Babesia microti, along with species of
Bartonella bacteria and Anaplasma
phagocytophilum (a tick-transmitted parasite to humans and
other animals).

The huge impact on infection chances rivaled more traditional
factors, such as season. For instance, infection risks from
certain parasites can change during the course of a year because
of their seasonal life cycles.

"The size of the effects was perhaps the most surprising," Telfer
said in an e-mail. "The impact of other parasites on infection
risk was essentially as significant as season."

Past interactions among bacteria or viruses – classified as
microparasites – are not unexpected. One well-known example is
the HIV
virus, which cripples the human immune system's defenses and
opens the door for other infections.

But researchers have typically focused on single parasites that
cause recognizable disease symptoms, or creepy effects, such as
mind control. They have paid less attention to the more
invisible web of interactions among parasites within the same
host.

A renewed look at the community of parasites living in a host may
change how experts decide to tackle infectious diseases. For
instance, a well-intentioned health policy that tries to
eliminate a certain parasite might just leave people vulnerable
to invasion by a worse parasite.

"If some human parasites are equally interactive, our current,
disease-by-disease
approach to modeling and treating infectious diseases is
inadequate," said Kevin Lafferty, a parasite ecologist at
the University of California in Santa Barbara, in a perspectives
article on the new study in the Oct. 8 issue of the journal
Science.

As for Telfer and her colleagues, they hope to next study the
interactions between tiny microparasites and larger
macroparasites, such as nematode worms. Their research will still
focus on voles, but Telfer has urged new studies that look at
co-infection in humans.

"To improve our ability to predict and control disease in humans
we need to study parasite communities, rather than parasites in
isolation," Telfer said.